Harumi Aoki

Rising slope of moment rate functions: The 1989 earthquakes off east coast of Honshu

A series of large earthquakes occurred off east coast of Honshu, Japan, during the period of October to November 1989. Mechanisms were studied for the foreshock (M=6.5), the mainshock (M=7.1) and the two aftershocks (M=5.2 and 5.3) using the STS-I broadband records at station INU about 700km away from the epicenters. The lowpass filtered seismograms of these earthquakes show mutually very similar waveforms, in good agreement with those calculated for a known mechanism by an extended reflectivity method. The mainshock is 10 times as large as the foreshock and 2000 times as large as the aftershocks in terms of seismic moment. Assuming that the mainshock (or foreshock) is a collection of some elementary sources at a focal point and that aftershock seismograms represent the responses to such elementary sources, the broadband seismogram of the mainshock (or foreshock) for the first 60s was deconvolved with that of an aftershock to obtain the moment rate function. The moment rate functions of the mainshock and foreshock are both triangle-like with durations of 14 and 9s respectively. Their rising slopes are mutually very different, a greater slope is associated with the greater shock. This and the similar results for other pairs of earthquakes suggest that the rising slope of moment rate function is causaly related to the eventual size of seismic rupture.

Real-Time Data Exchange between University Networks for Microearthquake Observation in Japan

Real-time exchange of waveform data from neighboring seismograph networks is significantly advancing seismic monitoring in central Japan. Five observation centers are connected by leased circuits from NTT (Nippon Telegraph and Telephone Corporation), to exchange three to nine channels of seismic signals on a routine basis. Detection capability and accuracy of seismic parameters for events near network boundaries are considerably improved with this system. It can fill the observation gaps between networks, as verified at the Izu Peninsula and the Hida region, central Japan. Monitoring of seismic activity in the Izu Peninsula, for example, is greatly improved. By connecting local networks in this way, a regional observation system for microearthquakes has been established in central Japan.